1. Field of the Invention
The present invention relates to the manufacture of cyanuric acid by the heat treatment of urea. The present invention further relates to a novel continuous process for the preparation of cyanuric acid in high yields and high purity which obviates the conventional acid hydrolysis treatment of the crude cyanuric acid product.
2. Description of the Prior Art
Cyanuric acid, which is the desired end product of the present invention, has the empirical formula C.sub.3 H.sub.3 O.sub.3 N.sub.3. It is known to prepare cyanuric acid by a variety of procedures, the most common of which is to merely melt and heat, e.g., pyrolyze technical grade urea at atmospheric pressure above the temperature range at which it decomposes (e.g., 180.degree. C. to 300.degree. C.).
In preparing cyanuric acid by the pyrolysis of urea, however, it is difficult to minimize the production of undesired autocondensation by-products. It is also difficult to obtain the desired cyanuric acid product in good yield and high purity. A cyanuric acid product of high purity is especially important where it is to be utilized as a raw material for other products. Hence, it is necessary to obtain a commercial product substantially free of pyrolytic autocondensation products of urea, and particularly free of amides of cyanuric acid, chiefly ammelide and ammeline.
The pyrolysis of urea may be carried out either in a dry state or in the presence of various inert solvents such as described in U.S. Pat. Nos. 3,065,233; 3,117,968; 3,164,591 and 3,563,987. More recent U.S. Pat. No. 3,810,891 teaches the preparation of cyanuric acid by the pyrolysis of urea or a urea pyrolyzate in an inert polyether solvent at temperatures from about 160.degree. C. to about 220.degree. C. while sparging with an inert gas.
For the most part, however, the use of inert solvents during the pyrolysis of urea still does not achieve a cyanuric acid product of adequate purity for most commerical purposes. To obtain a highly purified product, therefore, it is customary in the art to treat crude cyanuric acid in a strong acid bath with heating, e.g. 3-15% sulfuric acid or hydrochloric acid at nearly 100.degree. C. This acid treatment selectively hydrolyzes the acid-insoluble cyanuric acid amides and converts them to cyanuric acid. Such procedures generally require several hours to complete and further require special acid-resistant holding tanks and centrifuges to hold the acid bath and separate the digested cyanuric acid from the mineral acid. This refining step is known in the art as the "acid digestion step" and is described in U.S. Pat. No. 2,943,088. Suitable acids disclosed in U.S. Pat. No. 2,943,088 for the acid digestion step are hydrochloric, hydrobromic, sulfuric, nitric, phosphoric and various organic acids.
Certain prior art procedures have avoided the acid digestion step by pyrolyzing urea under subatmospheric pressure conditions. Such procedures, however, require additional energy output to maintain the system under vacuum and further can result in substantial loss of urea reactant due to sublimation.
U.S. Pat. No. 2,790,801, which issued Apr. 30, 1957, discloses a relatively simple process for preparing cyanuric acid from urea. Urea is mixed with sulfuric acid and the mixture is heated to about 200.degree. C. Although yields above 90% are said to result, the patentees in U.S. Pat. No. 2,790,801 gave no recognition to ammelide and ammeline impurities which necessarily occur in a product so produced. An acid digestion step, therefore, would still be required to achieve the levels of cyanuric acid purity to meet modern commercial standards. The latter standards are especially rigid if the cyanuric acid is to be later chlorinated.
For the foregoing reasons, it would be desirable to provide an improved process wherein a crude cyanuric acid product can be obtained in high yields and purity from a single pyrolysis reaction without the necessity of maintaining the process under vacuum or subjecting the crude cyanuric acid product to an acid digestion step. This is the primary object of the present invention. Another object of this invention is to provide a crude cyanuric acid product suitable for swimming pool use and for subsequent chlorination with the need for only a minimum of further purification.